people have wondered why Lucas added the factor of the midichlorians, and i am going to tell you why they are there. I have reliable source, of course i can not reveal who they are, but here it goes:
Remeber the Emperor's Royal Guard, and how they were force sensitive, well in all likelyhood, they were clones. As you might have guessed you can not clone spirituality, however, if there was a physical aspect to the force, then it could be cloned as well. This is precisely why Lucas has added this physical factor. The reason it is not mention in ANH, ROTJ, ESB, is that there were no clones that truly mattered because the empire had won,a dnt eh jedis were thought dead. There was no need to test luke because they knew that the force flows from one generation to another. Pass it on! It is information that i feel may clear George's Name on the first film, specifically in reference to the midichlorians, and the lack of spirituality in the force.
This factor will be explored in the SWII, speciffically in regard to the palpatine's personal guards, and creration of Dark jedi.